This invention relates generally to registration control methods and apparatus, and more particularly to a method and apparatus for automatic registration control of operations performed on a moving web by successive stations in a multi-station processing system.
In some types of multi-station processing systems wherein successive processing operations are performed by different processing stations on a running web of material, it is necessary that the locations on the material at which the successive processing operations are performed have a predetermined relationship to one another. In multi-color printing, for example, each of several printing stations applies a different color ink in a predetermined pattern to a substrate, e.g., paper that moves through the printing press. The patterns are superimposed to form the desired image, and the shape of each pattern and the density of ink deposited by each station are selected so that when the colors overlap a complimentary color is produced. Although all of the colors of the visible spectrum can be formed using the three primary colors red, yellow and blue, printing presses for high quality multi-color printing may, in general, have five printing stations. Three are used for the primary colors; one is used for black; and one is available for a special color that may be difficult to obtain by combining the three primary colors or where it is difficult to maintain the color match during an entire run. To obtain high quality resolution in the final product, it is necessary that the patterns printed by each station be precisely aligned. This alignment is called registration.
Registration control, which may be either manual or automatic (or perhaps both) involves controlling the various stations so as to achieve and maintain proper alignment. In a printing press, each station may print, along the waste edge of the paper, registration marks, e.g., crosshairs, corresponding to the location of each color. If registration is correct, the crosshairs will overlap. If a particular color is offset, an operator can manually energize a motor which shifts the relative position (or phase) of the rotating print platen of that station with respect to the other stations until registration is obtained. One color, selected arbitrarily, is generally designated as a reference color and the other stations are aligned to it. Once the alignment has been set, the press is run. Since misalignment may occur during a run, the operator must check the alignment from time to time. Misalignment must be detected with the press in operation, and the offset must be estimated visually. Although an experienced operator may be able to estimate the offset with an acceptable degree of accuracy, he may have to make several adjustments before an acceptable product is obtained. Moreover, it is desirable to maintain precisely an alignment of the order of 0.002 inch, for example, which is beyond the capability of most operators.
Generally, automatic registration systems may be one of two types--those that measure the actual position of a work-applying member at each station and adjust the positions of the members with respect to each other or with respect to some reference, e.g., a reference mark on a workpiece that moves from one station to the next, or those that compare the locations on the workpiece at which each processing operation is performed and adjust the work-applying members until the locations of the processing operations have a predetermined relationship, without regard to the actual positions of the work-applying members. In multi-station printing presses, systems of the first type may employ magnetic sensors for measuring the angular positions of the rotating print platens and photoelectric sensors for detecting the locations of reference marks on the workpiece. Systems of the second type may employ only photoelectric sensors for detecting the positions of spaced registration marks applied to the workpiece by each station.
The registration accuracy obtainable with systems of the first type is dependent upon the accuracy with which the positions of the work-applying members can be determined and the accuracy with which their positions relative to the reference can be established. Moreover, these systems assume that the locations of the processing operations on the workpiece are precisely determined by the positions of the work-applying members. In printing presses, for example, misalignment of the printing plates on the rotatable print platens, mechanical wear, or misplacement of the sensors may all produce registration errors. Systems of the second type may avoid some of these problems. However, their accuracy is still limited by the accuracy with which the registration mark positions on the workpiece can be determined.
In multi-station printing presses, registration marks may be sensed by projecting a spot beam of light onto the running web and detecting changes in the amount of light reflected as the marks pass through the beam. Two or more such sensors may be employed for detecting the positions of two or more side-by-side marks. To avoid displacement errors, the sensitivity of each sensor must be held constant, and care must be taken to avoid light from one sensor spilling over to an adjacent sensor and causing interference. Moreover, since different color marks reflect different amounts of light, fixed threshold sensors will produce output signals at different relative positions of the marks with respect to the sensors, causing errors in the determinations of mark positions. One known system attempts to avoid this by using a first sensor to measure the contrast ratio as each mark passes through its light beam and uses this ratio for automatically setting the threshold of a spaced second sensor employed for detecting mark position. This approach still requires that the sensitivities of the sensors be held constant and that interference between the sensors be avoided.
Known automatic registration systems also have other problems. Dimensional changes in the running web occasioned by stretching and shrinking of the web as it passes through the press cause the spacings between registration marks to change and may produce errors. Moreover, there is a time lag between the time at which a registration correction is made and the time at which the effect of the correction can be detected, during which the system must idle. This time lag is dependent upon the operating parameters of the processing system, e.g., operating speed, which may be variable. Accordingly, problems arise in synchronizing automatic registration systems to processing system parameters.